| Summary: | 1. Single fast and small bundles of slow fibres were isolated from the muscles of an elasmobranch (dogfish, Scyliorhinus canicula ) and an Antarctic teleost ( Notothenia neglecta ). A third fibre type present in the dogfish (superficial fibre) was also isolated. Fibres were chemically skinned with a non-ionic detergent. 2. Tension generation and ATPase activity were measured during isometric activations. ATPase activity was estimated by measuring the release of ADP into the experimental solutions using high performance liquid chromatography. 3. In the dogfish fibre types, both tension and ATPase activity increased in the order superficial < slow < fast, even after corrections were made for differences in myofibrillar density. The economy of isometric contraction (tension/ATPase activity) was 50-60% higher in the slow and superficial fibres than in the fast. 4. In the Antarctic species, both tension and ATPase activity of the fast fibres were higher than those of the slow fibres, and the slow fibres were 30% more economical than fast fibres. After correction for differences in myofibrillar density, tensions were very similar. 5. The results are discussed with reference to the energy supply, recruitment pattern and function of the various fibre types.
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